The present invention relates to improving external light extraction from light emitting diodes (LEDs) and in particular relates to improving the external light extraction from white light emitting diodes formed in the Group III nitride material system
A light emitting diode is a photonic device that emits light under the application of current. The current moves carriers (electrons or holes) across a p-n junction (or some functionally equivalent structure). When the injected carriers recombine (with holes or electrons respectively), they can emit light as the manner of releasing the energy from the recombination events.
In accordance with well-understood quantum theory, the energy of the recombination event is fundamentally established by the bandgap of the semiconductor material. Accordingly, the energy of the recombination can never exceed the bandgap, although in some cases depending upon doping and other characteristics, the energy may be somewhat less than the bandgap.
The light that falls within the visible spectrum has wavelengths of between about 390 and 770 nanometers (nm). When converted into energy equivalents using well-established relationships, this means that visible light can be created by recombination events in semiconductor materials that have bandgaps of at least about 1.8 electron volts (eV). As a result, diodes formed from some widely used semiconductor materials—e.g., silicon (1.1 eV)—cannot produce visible light.
Light emitting diodes have many of the same favorable characteristics as other solid-state devices. They tend to be reliable, physically robust, exhibit long lifetimes, and can be made at relatively low cost in large quantities. Accordingly, light emitting diodes have become extremely common in everyday use, particularly as indicator lights and in other lower-brightness applications.
More recently, illumination rather than just indication, has become a consistently attractive application of light emitting diodes. Illumination, however, typically requires white light. Many of the common materials (gallium phosphide, indium phosphide, gallium arsenide) used for light emitting diodes, however, have bandgaps that can only to produce lower-energy yellow or red light.
As is also well understood in the fields of optics and color perception, white light does not consist of a single frequency or narrow band of frequencies. Instead, white light represents a combination of frequencies that together produce the color white for the human eye.
Accordingly, two typical approaches are used to produce white light emitting diodes. In the first, individual red-emitting, green-emitting and blue-emitting diodes are combined in close proximity to generate white light from the combination of the red, green, and blue frequencies. In the second approach, a blue light emitting diode is used in combination with a phosphor, often a yellow-emitting phosphor, that emits in response to excitation from the blue LED. The combination of the blue and yellow frequencies can produce white light.
Accordingly, both approaches require blue light emitting diodes. In turn, because the wavelengths (frequencies) which the human eye perceives as blue fall in the range of between about 455 and 492 nanometers, only semiconductor materials with equivalent bandgaps of at least about 2.4 eV can produce blue light. Accordingly, the typical candidate materials for blue light emitting diodes (and thus for white-emitting diodes) are silicon carbide (SiC) and the Group III nitrides.
As between these two material systems, the Group III nitrides have the advantage of being direct emitters (silicon carbide is an indirect emitter). In a direct emitter, most of the energy from the recombination is emitted as the photon. In an indirect emitter, some of the energy is emitted as a photon, but most is emitted as vibrational energy. Accordingly, all other factors being equal, a direct emitter will produce light more efficiency than an indirect emitter and thus the Group III nitrides are increasingly preferred over silicon carbide for blue light emitting diodes.
For a number of reasons that are generally well understood in this art, when materials such as the Group III nitrides are used to produce a light emitting diode, the fundamental structure is a p-n junction formed between respective p-type and n-type layers of Group III nitride material. Generally speaking, high quality epitaxial layers can be grown in well-controlled systems.
Epitaxial layers, however, must be grown upon some type of substrate crystal. Appropriate large crystals of Group III nitrides, however, are difficult or commercially impractical to obtain and use as substrates for Group III nitride epitaxial layers. Accordingly, sapphire and silicon carbide are the two most typical choices for Group III nitride growth substrates.
Sapphire (Al2O3) offers transparency, which, as discussed further herein, can be a favorable factor in diode performance. Nevertheless, sapphire is insulating and cannot be conductively doped.
Silicon carbide has a better lattice match with the Group III nitrides than does sapphire thus reducing the difficulty of crystal growth and the resulting stress or strain on the epitaxial layers. Silicon carbide can also be conductively doped, giving more options for device design than does sapphire. Colorless silicon carbide is, however, harder to obtain than is colorless sapphire.
Nevertheless, all other factors being equal, higher-quality Group III nitride epitaxial layers, and thus higher-quality junctions and diodes, are formed on silicon carbide substrates rather than sapphire substrates.
Light emitting diodes, however, are used in context rather than in the abstract. As noted earlier, in many cases a white-emitting LED consists of a blue light emitting diode combined with a phosphor. Because a phosphor is typically a particulate mineral, it is usually dispersed in a polymer lens that covers and packages the blue-emitting LED chip. Thus, the structure, composition and orientation of the lens and phosphor will affect the overall performance of the packaged LED.
Furthermore, better brightness results have recently been obtained from Group III nitride epitaxial layer junctions that are first produced on a growth substrate of silicon carbide, then joined using a metal bonding system to a mounting substrate (often of a material other than SiC). The original SiC growth substrate is then removed to produce a diode structure with Group III nitride epitaxial layers on the metal bonding system, which in turn is on the mounting substrate.
For example, the EZBRIGHT™ diodes available from Cree, Inc., Durham, N.C., USA, the assignee of this application, are formed of epitaxial layers of Group III nitride mounted on a metal bonding structure that in turn joins to a silicon substrate. Silicon is used as the mounting substrate because it is well understood and widely available at relatively low cost.
Silicon, however, tends to absorb light at the frequencies produced by Group III nitride emitting structures. Accordingly, for any given number of photons produced by the light emitting diode, some of them will strike, and be absorbed by, the silicon mounting structure. Every photon that is absorbed in this manner reduces the external quantum efficiency of the diode.
As brief background, the efficiency of a diode; i.e., the amount of light it produces based on the amount of current applied, depends on two basic factors: first, the efficiency with which the diode creates photons from a given amount of current; and second, the efficiency with which the photons that are created actually leave the diode and its package and can be observed or perceived.
Some Group III nitride diode structures formed on silicon mounting substrates tend to be less bright than otherwise similar diodes formed on sapphire. Nevertheless, as noted above, Group III nitride layers grown on silicon carbide are, all other factors being equal, often better than those grown on sapphire.
Accordingly, the initial advantages of higher-quality Group III nitride layers grown on silicon carbide substrates tend to disappear when those layers are placed on certain light-absorbing mounting structures and incorporated into LED lamps using phosphors to create white light.
Therefore, a need exists for light emitting diode structures that maintain the initial advantages of Group III nitride layers grown on silicon carbide substrates even after the growth substrate has been removed and after the layers have been positioned on a mounting structure and after the structure has been formed into a lamp with a lens and a phosphor.